This invention relates to fastening, and more particularly to a probe and method for measuring the countersink depth of countersunk holes for fasteners, and also to a probe and method for measuring the protrusion height of fasteners above the surface of a workpiece.
A commercial jet liner subject to a high rate of take off and landing cycles is exposed to a stress cycle caused by repeated pressurizing and depressurizing of the fuselage as the plane ascends to cruising altitude and then descends to land. The stresses which are basically hoop stresses on the fuselage caused by internally pressurizing the fuselage for passenger comfort, and radial stresses caused by differential pressure on the fuselage panels, tend to be concentrated at the rivet points and are transmitted between the rivets and the rivet skin at the margins of the rivet holes.
To maximize the service lifetime of such an airplane, it is desirable to ensure that the rivet holes be designed to maximize the fatigue resistance of the skin in the region of the rivet holes to minimize or prevent the formation of fatigue cracks in the airplane skin. This is particularly important in the fuselage skin which is normally made fairly thin to minimize weight.
A Briles rivet, as shown in U.S. Pat. No. 4,159,666, includes a head having a cylindrical section ("barrel") and a tapered section, and a cylindrical shank projecting from the tapered section. The purpose of the barrel is to seal the rivet head in the counterbore of the rivet hole when the rivet is upset to prevent entrance of water between the rivet head and the rivet hole which could cause corrosion and staining of the exterior of the fuselage. An even more important function of the Briles rivet head barrel is to cold work the skin material in the marginal regions of the rivet hole to strengthen and harden the material to improve the fatigue resistance to cracking. Without this cold working, the fuselage skin would have to be substantially thicker to provide the desired fatigue resistance.
Use of the Briles rivet on a thin skin fuselage panel requires that the depth of the hole counterbore and the countersink be precisely accurate. If the countersink is too deep, it could form a "knife-edge" condition at the bottom of the countersink and the bottom or inside face of the skin which can lead to the initiation of a fatigue crack at the "knife edge". If the countersink is too shallow, the barrel of the rivet head will protrude from the counterbore of the rivet hole into the airstream and create unacceptably high drag. For this reason, rivet holes are drilled with extremely accurate tooling to ensure that the holes are the correct diameter and depth.
Even with the precision tooling in use for drilling accurate holes, it is desirable for statistical process control and other purposes to measure the depth of a rivet hole countersink. The most widely used procedure is to insert a rivet in the rivet hole and measure the protrusion of the rivet head out of the hole before the rivet is upset. This technique occasionally fails to detect a too-deep countersink when the thickness of the rivet head itself exceeds the tolerance and conceals the fact that the rivet hole countersink is too deep. As a back-up quality control technique, the height of the rivet protrusion from the rivet hole is ascertained after the rivet has been upset. If it is flush with or below the skin surface, the rivet hole countersink is presumed to be too deep and a repair is made. However, a too-deep hole and a too-thick head will combine to defy detection by these two measurement techniques.
To ensure that the rare coincidence of a too-deep rivet hole and a too-thick rivet head do not go undetected, it would be desirable to be able to measure the depth of a rivet hole countersink directly. It would seem that the depth measurement could be made easily merely by inserting a probe into the hole and measuring the depth of penetration to the bottom of the countersink, but this seemingly straightforward task is actually more complicated than it first appears. The depth of penetration of the probe is a function of the probe diameter, since the surface it contacts is a tapered surface. Also, the hole countersink is blended into the counterbore and the shank bore with radiused transition regions which should not be contacted for depth measurement. Finally, the tolerances of the countersink angle, the skin thickness, the counterbore and shank bore diameters and the probe diameter, as well as the measurement accuracy of the instrument transducer must be considered in individual and stacked fashion to ensure that a hole depth measured at the limit of all the tolerances involved and appearing to fall within the permitted range does in fact leave sufficient shank bore depth to the enable the rivet hole to have the required fatigue resistance.
A typical riveted structure, such as an airplane, has countersunk rivets of numerous sizes, so a countersink depth gauge of maximum usefulness should have the capability to measure more than one size rivet hole, and preferably all the rivet holes that are used on the structure. Because of the requirement that the probe directly contact the tapered countersink surface, it would seem to be a logical solution to use a separate probe for each size rivet hole. However, a depth gauge provided with replacable probes must be accurate for all size holes, and that accuracy must not be compromised by the act of exchanging probes for measuring different size holes.
The protrusion of a fastener head above the surface of the airplane is an important factor in the performance and structural integrity of the airplane structure. If the fastener head, of rivets in particular, is flush with or below the skin surface, there is no certainty that there is adequate interfacial pressure under the fastener head. If the fastener head protrudes excessively above the skin surface, the aerodynamic drag becomes significant. The trends in fastener head protrusion between the two tolerance limits can be a useful analytic tool for manufacturing quality control. For these reasons, a gauge for measuring fastener head protrusion would be a valuable tool, especially if that gauge could be based on the same tool used to measure the countersink depth of the fastener head.